Quoted from yaksplat:I can't think of any reason that this screw needs to be nylon.
It doesn't have to be.
Nylon would be quieter when the plunger hits it. And the end won't peen over like a metal one, should you ever want to remove it, which would make unscrewing it hard if not impossible. And you won't wear a spot on the bottom of the plunger.
LTG : )